All cells store neutral lipids (triacylglycerol and cholesteryl esters) in discrete intracellular neutral lipid storage droplets. In the general cell population, very small droplets transiently sequester fatty acids which are used for membrane biosynthesis and as a source of energy. In adipocytes, which store the main bodily energy reserves, fatty acids are mobilized from very large, triacylglycerol-rich, droplets and exported to other tissues. In steroidogenic cells, the droplets contain primarily cholesteryl esters which are the precursors for steroid hormone synthesis. Our research focuses on the surface composition of droplets and the mechanism by which lipids are both deposited and withdrawn. We have found that droplets surfaces are coated with two related proteins, perilipin and adipocyte differentiation-related protein (ADRP). Despite its name, ADRP is expressed ubiquitously and occurs on the surface of droplets in all cells examined, whereas the perilipins are expressed primarily in adipocytes and steroidogenic cells. Although neutral lipids are synthesized on the endoplasmic reticulum, perilipins and ADRP are synthesized on free polysomes, indicating that these proteins are not involved the the formation of nascent droplets as components of the lipogenic complex; rather, they appear to be involved in a sequestration or packaging step that occurs subsequent to neutral lipid synthesis. Moreover, lipid deposition is required for the expression of perilipins and ADRP; association with neutral lipid is necessary to stabilize these proteins. Previously we demonstrated that ectopic expression of perilipin A, the major adipocyte isoform, increases neutral lipid deposition in several cell lines. Recent findings indicate that the increase in lipid reflects resistance to lipid hydrolysis rendered by the perilipins at the droplet surface. This property of the perilipin A may also explain a change in the surface composition during the maturation of droplets in adipocytes; nascent droplets in differentiating cells are coated initially with only ADRP, but as the droplets increase in size they lose the ADRP and acquire perilipins. Physiological stimulators of lipolysis increase cAMP and activiate protein kinase A. Unlike ADRP, perilipins are phosphorylated by protein kinase A, which also phosphorylates hormone-sensitive lipase, the rate-limiting enzyme of neutral lipid hydrolysis in adipocytes. Phosphorylation of perilipin results in a disruption of the lipid droplet surface, which probably facilitates lipid hydrolysis. Thus, the switch from ADRP to perilipin in adipocytes may provide resistance to inappropriate lipid hydrolysis in the absence of a physiological stimulus.